The present invention relates to measuring apparatuses and measuring methods that use Brillouin scattered light.
With the development of optical fiber communication, distributed optical fiber sensing using the optical fiber itself as a sensing medium has been actively researched. A representative example of distributed optical fiber sensing is optical time domain reflectometry (OTDR) in which an optical pulse is input from one end of the optical fiber and light backscattered within the optical fiber is measured relative to time. Examples of backscattering within the optical fiber include Rayleigh scattering, Brillouin scattering, and Raman scattering. Among these examples, one that measures spontaneous Brillouin scattering is called Brillouin OTDR (BOTDR) (for example, see “T. Kurashima et al., “Brillouin Optical-fiber time domain reflectometry”, IEICE Trans. Commun , vol. E76-B, no. 4, pp. 382 to 390 (1993)”).
Brillouin scattering is observed at a location shifted toward the Stokes side or the anti-Stokes side by a frequency of about a GHz relative to the central frequency of the optical pulse input to the optical fiber, and a spectrum thereof is called a Brillouin gain spectrum. A frequency shift amount and a spectral line width of a Brillouin gain spectrum (BGS) are called a Brillouin frequency shift (BFS) and a Brillouin line width, respectively, and vary depending on the material of the optical fiber and the wavelength of the optical pulse input to the optical fiber. For example, in a case where an optical pulse with a wavelength of 1.55 μm is input to a silica-based single-mode optical fiber, it is reported that the −BFS is about 11 GHz and the Brillouin line width is about 30 MHz.
A BFS is known to change linearly at a rate of about 500 MHz/% relative to strain in the optical fiber. When this is converted into tensile strain and temperature, 0.049 MHz/pc and 1.0 MHz/° C. at 1.55 μm wavelength are obtained, respectively.
Accordingly, in BOTDR, strain and temperature distribution in the longitudinal direction of the optical fiber can be measured. Therefore, BOTDR is attracting attention as a technology for monitoring large-scale structures, such as bridges and tunnels.
Since BOTDR measures the spectrum waveform of spontaneous Brillouin scattered light occurring within the optical fiber, heterodyne detection of separately-prepared reference light is normally performed. The intensity of spontaneous Brillouin scattered light is smaller than that of Raleigh scattered light by two to three digits. Therefore, heterodyne detection is effective for improving minimal light-receiving sensitivity.
BOTDR in the related art will be described with reference to FIG. 6 (for example, see JP 2001-165808A). FIG. 6 is a schematic block diagram of an optical-fiber strain measuring apparatus in the related art.
Continuous waveform (CW) light output from a light source 112 is bifurcated into two light beams by an optical coupler 142. One of the two bifurcated light beams is used as reference light, whereas the other light beam undergoes a frequency shift equivalent to a Brillouin frequency by an optical frequency shifter 143 and is subsequently made into a pulse-like probe light by an optical pulse generator 114.
This probe light is input to an optical fiber 100 to be measured (i.e., a fiber to be measured) via an optical coupler 120. Brillouin backscattered light from the optical fiber 100 to be measured is multiplexed with the reference light in an optical coupler 150 and subsequently undergoes heterodyne detection by a receiver 160 constituted of a balanced photodiode (PD) 162 and a field-effect-transistor (FET) amplifier 164.
Since the probe light is frequency-shifted by about a Brillouin frequency by the optical frequency shifter 143, the frequency of a beat signal generated as a result of heterodyne detection is low. After using a mixer 170 and an electric filter 178 to downshift the frequency of the beat signal, the power and the amplitude of an intermediate frequency (IF) signal obtained as a result of square-law detection or envelope detection are measured by a detector circuit 172. The results are sent to a signal processor 174.
Because BOTDR deals with information about frequency spectrum distribution in the longitudinal direction of the optical fiber, three-dimensional information related to time, amplitude, and frequency has to be acquired. A method for acquiring three-dimensional information related to time, amplitude, and frequency in BOTDR will be described with reference to FIG. 7. FIG. 7 is a schematic diagram illustrating the method for acquiring three-dimensional information related to time, amplitude, and frequency in the optical-fiber strain measuring apparatus in the related art. In order to measure the entire Brillouin frequency spectrum in the technology disclosed in JP 2001-165808A mentioned above, two-dimensional information related to time t and amplitude I is acquired by sweeping a frequency f of a local oscillation electric signal source 183.